1. Field of the Invention
This invention relates to a drive circuit for a magnetron used in a pulse-modulated radar, for example.
2. Description of the Prior Art
A pulse-modulated radar (hereinafter referred to simply as the pulse radar) is a radar which determines the distance to a target from the time that elapses after transmitting pulsed waves toward the target before a portion of the pulsed waves reflected by the target is received. Given the velocity of light, c, and the elapsed time xcex94t between transmission and reception, the distance D to the target is calculated as follows:
D=cxcex94t/2
Important factors for judging the performance of the pulse radar are such characteristics as bearing discrimination, maximum and minimum detecting ranges, as well as range discrimination. The range discrimination is defined as a minimum distance R at which two targets placed on the same bearing with respect to a radar antenna can be displayed separately. Since radio waves propagate with a speed of about 300 m/xcexcs, they make a round trip of about 150 m within a period of 1 xcexcs. Thus, there is a relationship expressed by the following equation between pulselength xcfx84 (xcexcs) and the range discrimination R:
R=300xcfx84/2=150xcfx84(m)
Basically, the range discrimination is determined by pulselength, that is, the smaller the pulselength, the better the range discrimination and short-range detection.
The construction of a pulse radar is now generally described. The pulse radar is constructed of a magnetron for generating microwaves, a magnetron drive circuit for driving the magnetron, an antenna, a receiver circuit and other electronic components. FIG. 3 is a circuit diagram generally showing the configuration of a conventional magnetron drive circuit 101. As depicted in FIG. 3, a pulse transformer 11 is used in the conventional magnetron drive circuit 101. One end of a primary winding 12 of the pulse transformer 11 is connected directly to a power source V and grounded through a capacitor 20 while the other end of the primary winding 12 is connected to a drain of a switching n-channel metaloxide-semiconductor field effect transistor (NMOSFET) 21 (hereinafter referred to as the switching FET 21). A source of the switching FET 21 is directly grounded and its gate is grounded through a resistor 22. An absorption resistor 23 is normally connected between both ends of the primary winding 12. On the other hand, one end of a secondary winding 13 of the pulse transformer 11 is connected to a cathode of the magnetron (not shown) while the other end of the secondary winding 13 is connected to an anode of the magnetron.
In the magnetron drive circuit 101 constructed as described above, the switching FET 21 turns on when a transmission trigger having a specific pulselength is fed into the gate of the switching FET 21. As a result, a high-voltage pulse having the same pulselength is generated. When the high-voltage pulse is applied to the magnetron, it oscillates and produces an extremely high-power microwave output (transmission pulse), which is radiated from the radar antenna (not shown).
It has been recognized that the combination of the aforementioned magnetron drive circuit 101 and the magnetron of the prior art has a problem known as a transmission missing phenomenon which occurs as follows. If the rising edge of a pulse for driving the magnetron is made too sharp to obtain a narrow pulselength, the magnetron will fail to oscillate, resulting in an inability to generate the transmission pulse.
Also, it is generally needed to sharpen the falling edge of the transmission pulse to obtain a narrow pulselength. In the conventional magnetron drive circuit 101, the absorption resistor 23 is added as stated above to decrease residual energy left in the pulse transformer 11 to zero level in a short time. The absorption resistor 23, however, acts as an extra load in a rising period of the transmission pulse. Therefore, the resistance value of the absorption resistor 23 can not be made so small that it is impossible to obtain so sharp a falling edge of the transmission pulse.
FIG. 4 is a diagram showing the waveform of an input pulse fed into the conventional magnetron drive circuit 101 (bottom) and the waveform of a transmission pulse applied to the magnetron (top). Although the transmission pulse ideally should have a rectangular shape, the actual transmission pulse has a sawtooth shape as depicted in FIG. 4.
The invention is intended to provide a solution to the aforementioned problems of the prior art. Accordingly, it is an object of the present invention to provide a radar signal generator for producing a magnetron driving signal so that the aforementioned problems will be solved.
Accordingly, it is another object of the invention to provide a magnetron drive circuit which drives a magnetron to produce a generally rectangular-shaped narrow transmission pulse having sharply shaped rising and falling edges.
According to the invention, a magnetron drive circuit comprises a nonlinear load circuit, which becomes ON at a voltage approximately equal to a voltage at which a magnetron begins to oscillate, is connected to a secondary winding of a pulse transformer for generating a pulse for driving the magnetron in parallel with the magnetron.
This construction ensures that the flow of electrons from a cathode to an anode of the magnetron properly oscillates in an initial stage of oscillation. In other words, the construction of the invention helps prevent the so-called transmission missing phenomenon which could occur if the flow of electrons reaches the anode of the magnetron resulting in a failure of oscillation. Since the magnetron used in a pulse-modulated radar begins to oscillate typically at around 80% of a peak point of applied voltage, it is preferred to reduce the rate of increase of the magnetron input voltage within a time period during which the voltage applied to the magnetron rises from 80% to 100% of the peak voltage. On the other hand, because the transmission missing phenomenon does not occur even if the applied voltage is rapidly increased until the magnetron begins to oscillate, the rising edge of the magnetron input voltage is sharpened during a pre-oscillation period of the magnetron to obtain a narrow transmission pulse width.
In one aspect of the invention, the nonlinear load circuit includes a diode which breaks down at a voltage approximately equal to the voltage at which the magnetron begins to oscillate. The nonlinear load circuit of this feature can be produced by using an element easily available on the market.
In another aspect of the invention, the nonlinear load circuit is configured by connecting a parallel circuit, which is formed of a series circuit including a first resistor having a resistance approximately equal to the rated internal impedance of the magnetron and a capacitor having a specific capacitance and a second resistor having a resistance corresponding to two to three times the rated internal impedance of the magnetron, to the aforementioned diode in series.
In this construction, the first resistor, the capacitor and the second resistor together act as a temporary load, the capacitor serving as a particularly large load, when the diode has broken down. This helps decrease the rate of voltage rise close to the voltage at which the magnetron begins to oscillate, making it possible to prevent the transmission missing phenomenon in a more reliable manner.
In another aspect of the invention, the magnetron drive circuit further comprises a residual energy absorption circuit for absorbing residual energy left in the pulse transformer by short-circuiting one of its windings approximately at the same time as the voltage level of a transmission trigger fed into a primary winding of the pulse transformer begins to fall.
The residual energy absorption circuit absorbs the residual energy left in the pulse transformer without producing adverse effects on the shape of the magnetron transmission pulse during its rising period. In addition, since the residual energy absorption circuit absorbs the residual energy approximately at the same time that the residual energy in the pulse transformer becomes no longer needed, it is possible to make the falling edge of the transmission pulse sharper than that obtained with the prior art.
In another aspect of the invention, a magnetron drive circuit comprises a residual energy absorption circuit for absorbing residual energy left in a pulse transformer for generating a pulse for driving a magnetron by short-circuiting one of its windings approximately at the same time as the voltage level of a transmission trigger fed into a primary winding of the pulse transformer begins to fall.
Since the residual energy absorption circuit efficiently absorbs the residual energy left in the pulse transformer, it is possible to make the falling edge of the transmission pulse sharper than that obtained with the prior art.
In still another aspect of the invention, a magnetron drive circuit comprises a residual energy absorption circuit for absorbing residual energy left in a pulse transformer for generating a magnetron driving pulse to eliminate the need for an absorption resistor conventionally connected between both ends of a primary winding of the pulse transformer for absorbing the residual energy left in the pulse transformer.
The conventional absorption resistor connected between both ends of the primary winding has been associated with a problem that it acts as an extra load in a rising period of the magnetron transmission pulse, making it impossible to decrease the resistance value of the absorption resistor to a large extent and to obtain a sharp falling edge of the transmission pulse. In contrast, the aforementioned arrangement of the invention makes it possible to obtain a magnetron transmission pulse of a small pulselength having a generally rectangular waveform with sharply shaped rising and falling edges without producing adverse effects on the shape of the rising edge of the magnetron transmission pulse.
In further aspect of the invention, a radar signal generator comprises a magnetron for producing a radar signal, a signal generator for producing a generally rectangular waveform with a sharply shaped rising edge and a sharply shaped falling edge and supplying the signal from said signal generator to said magnetron.
In still further aspect of the invention, a radar signal generator comprises a magnetron for producing a radar signal, a pulse transformer having two primary windings and a secondary winding which is connected to said magnetron, a first switching semiconductor device having an input terminal and a output terminal which is connected to said magnetron, a second switching semiconductor device having an input terminal and an output terminal which is connected to one of said primary winding, wherein a transmission signal is supplied to the input terminal of said first switching semiconductor device and a generally rectangular signal is supplied to the input terminal of said second switching semiconductor device so that a generally rectangular waveform is produced at a secondary winding of said transformer with a sharply shaped rising edge and a sharply shaped falling edge.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.